However afaiui the FreeBSD manpage there are at least two other significant differences. "-I" changes xargs' behavior in multiple aspects:
#1 split only on line ends instead of also on space:
---------------------
ametzler@argenau:/tmp$ touch 'blah fasel'
ametzler@argenau:/tmp$ printf 'blah fasel\n' | xargs -I % ls %
blah fasel
ametzler@argenau:/tmp$ printf 'blah fasel\n' | xargs ls
ls: blah: No such file or directory
ls: fasel: No such file or directory
---------------------
#2 execute utility once for every line of input instead of making the commandline longer.

Afaiu the manpage "-J" is not supposed to make either of these two changes to xargs' default behavior, i.e. this should be the resulting behavior:
ametzler@argenau:/tmp$ printf 'blah fasel\nnonexist\n' | xargs -J % ls %
ls: blah: No such file or directory
ls: fasel: No such file or directory
ls: nonexist: No such file or directory
ametzler@argenau:/tmp$ printf 'blah fasel\nnonexist\n' | xargs -J % echo x %
x blah fasel nonexist

On top of that the patch seems to be buggy with respect to multiline input. Only the first line of input is actually passed on:
ametzler@argenau:/tmp$ printf 'blah fasel\nnonexist\n' | /tmp/findutils-4.2.27/xargs/xargs -J % echo x %
x blah fasel
x %

Sorry for the delay in following up.

cu andreas
PS: I have not got access to a FreeBSD system to actually test its behavior, my only source of input is the online manpage.

---------------------------
-I replstr
Execute utility for each input line, replacing one or more occur-
rences of replstr in up to replacements (or 5 if no -R flag is
specified) arguments to utility with the entire line of input.
The resulting arguments, after replacement is done, will not be
allowed to grow beyond 255 bytes; this is implemented by concate-
nating as much of the argument containing replstr as possible, to
the constructed arguments to utility, up to 255 bytes. The 255
byte limit does not apply to arguments to utility which do not
contain replstr, and furthermore, no replacement will be done on
utility itself. Implies -x.

-J replstr
If this option is specified, xargs will use the data read from
standard input to replace the first occurrence of replstr instead
of appending that data after all other arguments. This option
will not affect how many arguments will be read from input (-n),
or the size of the command(s) xargs will generate (-s). The
option just moves where those arguments will be placed in the
command(s) that are executed. The replstr must show up as a dis-
tinct argument to xargs. It will not be recognized if, for
instance, it is in the middle of a quoted string. Furthermore,
only the first occurrence of the replstr will be replaced. For
example, the following command will copy the list of files and
directories which start with an uppercase letter in the current
directory to destdir: